The present invention relates to an apparatus for thermal treatment wherein material to be treated is dehydrated (or dried) and thermally decomposed in a substantially oxygen-free state using mainly superheated steam. Here, the material to which the present apparatus is applicable is viscous and/or solid material excluding liquid and gaseous matter, for example, waste comprising one or more of garbage generated in general households, plants, restaurants or the like, used paper diapers, waste plastics easy to generate dioxin during combustion and other plastics, livestock excrement or human wastes, and sludge residue generated in sewage disposal plants. Further, the apparatus for thermal treatment according to the present invention can be applied to dehydration or thermal decomposition of materials or products comprising various kinds of organic and inorganic materials used at plants or the like.
Conventionally, for disposing of waste generated such as in plants and households, an incineration method has been adopted in view of a merit that a large quantity of waste can be treated at a relatively low cost. Moreover, various kinds of methods, such as an incineration method having a reaction zone where dioxin can be decomposed, a hydrothermal reaction method, a plasma reaction method and so forth have been proposed for disposing of waste containing toxic constituents to human bodies and an environment, such as CFC, PCB and trichloroethylene or the like, or waste plastics containing a chlorine element, since dioxin will generate by the simple incineration disposal. Further, for dehydrating materials and products at plants, dehydration using hot wind comprising air, nitrogen gas, or the like has been performed.
However, the above-mentioned conventional incineration method for disposing of waste has the following drawbacks. Since heat capacity generated during the incineration varies with calorie contained in the waste to be treated, it is difficult to control a furnace temperature. That is, in case of disposing of material containing much water or with a low calorie by the incineration, it is necessary to heat by a supplemental burner. On the other hand, in case of disposing of flammable material such as lumbers or papers by the incineration, the material burns by itself (namely, spontaneous combustion) without necessity of heating by the supplemental burner. However, it is necessary to control a charge of the material to be treated lest a temperature of a furnace should become too high. Further, when such waste with a high heat capacity is included as part of the waste to be treated, there is a problem such that the furnace temperature becomes high partially.
There is another treatment method in which the waste is first put into an oxygen-free furnace to be carbonized, and gas generated therefrom is subjected to a secondary combustion in another furnace. However, since it is difficult to heat the waste while stirring in the oxygen-free furnace, it takes much time to treat the waste, and further, when the waste contains chloride, it is necessary to install an apparatus additionally to treat generated dioxin.
Alternatively, to detoxify dioxin, CFC, PCB and trichloroethylene, the hydrothermal reaction method is extremely effective. However, since conditions of decomposition are high-temperature and high-pressure, such as the temperature is in a range of 300 to 450xc2x0 C. and the pressure is in a range of 100 to 250 kg/cm2, a vessel used in a decomposition apparatus needs to be resistant to high-temperature and high-pressure. Consequently, construction, maintenance and running costs of the apparatus are high and also, it is difficult to perform a continuous treatment. Therefore, the hydrothermal reaction method is not suitable for disposing of the above-mentioned waste. Furthermore, the plasma reaction method wherein the above-mentioned toxic substances are introduced into high temperature plasma for decomposition has a drawback that an apparatus and treatment costs are extremely high.
Next, in the dehydration of the material to be treated using air or nitrogen, a heat exchanger is necessary for raising the temperature of air or nitrogen to a high temperature. However, since heat capacities of air and nitrogen are small, a large heat exchanger is indispensable for treating a large amount of the material, thus when a treatment temperature exceeds 500xc2x0 C., the dehydration is not performed generally in the present situation.
The present invention is achieved in view of the above situation, and aims to provide an apparatus for thermal treatment using high temperature superheated steam performing dehydration (drying) and thermal decomposition of material (including waste), whereby a whole apparatus can be built relatively small and moreover, a treatment time can be shortened.
An apparatus for thermal treatment using superheated steam according to the present invention for attaining the above object comprises: a first rotary kiln having a first nozzle pipe therein for spouting a first high temperature gas, a first charge portion of material to be treated on one side thereof and a first discharge portion of the material dehydrated with the first high temperature gas on the other side thereof, the first high temperature gas comprising one or both of high temperature superheated steam and high temperature combustion exhaust gas; a second rotary kiln having a second nozzle pipe therein for spouting a second high temperature gas, a second charge portion of the material dehydrated by the first rotary kiln on one side thereof and a second discharge portion of the material carbonized with the second high temperature gas on the other side thereof respectively, the second high temperature gas comprising mainly high temperature superheated steam with a temperature higher than a temperature of the first high temperature gas; and a mechanism for discharging treated material provided to the second discharge portion of the second rotary kiln for discharging the carbonized material outside after lowering a temperature of the material in an oxygen-free state so that spontaneous combustion of the carbonized material may not occur in an atmosphere.
Thereby, the material can be thermally treated in two-steps at the first and second rotary kilns separately, i.e., dehydration and partial carbonization of the material can be carried out in the first rotary kiln, and perfect carbonization of the dehydrated and partially carbonized material can be carried out in the second rotary kiln. Further, if superheated steam is used as a heat source in the first and second rotary kilns, when the steam temperature is lowered, the superheated steam turns into water. Thereby, treatment of exhaust gas becomes easy. Accordingly, by using the two rotary kilns with different roles to treat the material, the thermal treatment can be carried out more evenly compared to a case where the thermal treatment of the material is carried out in only one rotary kiln, and further, efficient treatment becomes possible by properly dispersing heat energy into the two rotary kilns. Moreover, since the mechanism for discharging treated material is provided by which the material from the second rotary kiln is discharged outside after the temperature is lowered in an oxygen-free state so that the treated material may not burn by itself in an atmosphere, spontaneous combustion of the thermally treated material can be prevented when the material is discharged in the atmosphere. Further, it is preferable that the material to be treated is fractured to 2 cm or smaller (still preferably, 0.5-1.5 cm or so), and further to be granular. However, the present invention is not limited to a size and form of the material to be treated.
Additionally, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that the first rotary kiln is disposed above the second rotary kiln, the first discharge portion of the first rotary kiln having a chute for discharging the material from the first rotary kiln, a screw conveyor for feeding material which feeds the material discharged from the chute into the second rotary kiln being provided to the second charge portion of the second rotary kiln, the first discharge portion and the second charge portion being connected. Thereby, conveying the material from the first rotary kiln to the second rotary kiln becomes easy and the flow of the material becomes continuous. Moreover, a space necessary to install the apparatus becomes small.
Furthermore, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that the first and second nozzle pipes are extended in a fixed state from one side to the other side in kiln bodies of the first and second rotary kilns respectively, many nozzles for spouting high temperature gas being provided at intervals to the first and second nozzle pipes respectively. Thereby, high temperature superheated steam or high temperature combustion exhaust gas, namely the first high temperature gas, can be blown evenly in the kiln body of the first rotary kiln, and very high temperature superheated steam, namely the second high temperature gas, can be blown evenly in the kiln body of the second rotary kiln. Particularly since the kiln bodies rotate, the material is stirred. Thus, heat travels through the material within a relatively short time. Further, it is preferable that the first and second nozzle pipes are provided from one end (edge) toward the other end (edge) of the first and second rotary kilns respectively, the first and second nozzle pipes being in parallel with axes of the respective kiln bodies.
Still furthermore, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that the first high temperature gas supplied to the first rotary kiln is comprised of high temperature gas in a range of 200-700xc2x0 C. (still preferably, 200-400xc2x0 C.), and the second high temperature gas supplied to the second rotary kiln is comprised of superheated steam in a range of 400-1000xc2x0 C. that is higher than the temperature of the first high temperature gas used in the first rotary kiln. Thereby, it is possible to perform a two-step treatment with different heating temperatures in the first and second rotary kilns. Namely, in the first rotary kiln, dehydration of the material can be carried out mainly, and in the second rotary kiln, carbonization of the material can be carried out. Here, it is to be noted that the present invention is not limited to these temperature ranges.
Still furthermore, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that the mechanism for discharging treated material comprises a discharging screw conveyor which conveys the carbonized material being charged densely, an inside of the discharging screw conveyor being in an oxygen-free state by one or both of superheated steam and saturated steam flowing from the second rotary kiln into the discharging screw conveyor. Thereby, the carbonized material is discharged gradually from the second rotary kiln according to the treatment velocity, and the temperature of the treated material is lowered when the material passes through the discharging screw conveyor. Consequently, ignition and combustion of the treated material can be prevented. Further, since steam is used to keep the oxygen-free state, when the temperature of the material becomes below 100xc2x0 C., where the material does not burn, the steam turns into water. Therefore, gas that needs treatment will not be generated and the apparatus configuration is simplified. Accordingly, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that a downstream side of the discharging screw conveyor comprising a humidifying zone, the material charged from an inlet side of the discharging screw conveyor being humidified by the superheated or saturated steam liquefied in the humidifying zone, the material is discharged with a temperature lower than 100xc2x0 C. from an outlet of the discharging screw conveyor. Thereby, the material treated by the second rotary kiln can be discharged in a state of not burning after being properly humidified.
Still furthermore, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that the material to be treated is carbon-containing chemical compound, and the carbonized material (carbonized substance) of the carbon-containing chemical compound is discharged via the mechanism for discharging treated material. Thereby, the carbonized material can be used as such as activated carbon.
Still furthermore, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that furnace pressures of the first and second rotary kilns are respectively in a positive pressure state higher than atmospheric pressure during operation. Thereby, air is prevented from coming into the first and second rotary kilns and combustion of the material with the air can be prevented. Furthermore, the positive pressure state is preferably in a range where 10-100 mmAq or so is added to the atmospheric pressure. When the pressure is too high, a loss of heat energy is large, and when too low, air comes into the first and second rotary kilns partially.
Moreover, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that high temperature carbonization gas exhausted from one or both of the first and second rotary kilns is used as a part of a heat source to generate one or both of the first high temperature gas and the second high temperature gas. Thereby, with a little fuel or no fuel, the apparatus for thermal treatment using the superheated steam can be operated. Further, in the apparatus for thermal treatment using superheated steam according to the present invention, it is preferable that the carbonization gas being heated to a temperature of 800xc2x0 C. or higher in a combustion furnace for odor contained therein to be removed, one or both of the first high temperature gas and the second high temperature gas are generated by utilizing retained heat of combustion exhaust gas from the combustion furnace.